1. Field of the Invention
The invention generally relates to a vaccine for Brucellosis. In particular, the invention provides an antigenic composition comprising a recombinant, attenuated strain of Brucella suis with a deficiency in carboxyl-terminal protease (CtpA) activity.
2. Background of the Invention
Animal brucellosis is a disease affecting many domestic and wild life species. In male animals, this disease causes orchitis (inflammation of the testicles) and may eventually lead to sterility. In female animals, brucellosis causes abortion during the last trimester, retained afterbirth (retaining placenta in the uterus) and weakness in calves at birth. Brucellosis results from infection with bacteria belonging to the genus Brucella. On the basis of observed differences in host preference, which have been associated with certain phenotypic characteristics, this genus has been classified for convenience into six nomen species. These are associated with different principal hosts: B. abortus (cattle), B. canis (dogs), B. melitensis (sheep, goats), B. neotomae (desert wood rat), B. ovis (sheep) and B. suis (swine, reindeer). However, Brucella species typically can infect a wide variety of hosts, including humans.
Human brucellosis is a zoonotic disease, that is, it is readily passed to humans from other species. Infection in humans is normally acquired either through consumption of contaminated dairy and meat products or by contact with infected animal secretions. Human beings are susceptible to B. melitensis, B. suis, B. abortus and B. canis in a decreasing order. Brucellosis in humans is characterized by undulant fever, headache, cold sweats and general malaise. The disease can last from a few weeks to several years. If untreated, serious complications leading to death can occur.
Brucellosis among domestic livestock in North America has been largely controlled by using a combination of reliable and accurate diagnostic tests, removal of infected animals, and efficacious vaccines. However, this disease still exists among free-ranging wild life including feral swine (Sus scrofa). Infected wild life populations are the most likely source of transmission of brucellosis to humans, and for the possible reintroduction of this disease into domestic livestock. Feral swine populations are present in many regions of the world. Approximately 2 to 3 million feral swine are estimated to be present in the US alone, and feral swine populations in the southern portion of the US are known to be infected with B. suis. 
Brucellosis among US domestic pig populations is currently controlled by depopulation procedures, a less than ideal strategy. Further, swine brucellosis is recognized as a major threat to domestic pig production in other parts of world. So far, no vaccine has been extensively used or clearly been proven useful against this disease in swine. Therefore, it would be beneficial to have available a vaccine effective against brucellosis in feral and domestic swine.
B. suis was the first pathogenic organism weaponized by the U.S. military during the 1950 s. Today it constitutes a potential bioterrorism threat that could be targeted against military personnel, civilians, or food supplies. Early diagnosis of brucellosis is problematic, and the treatment regiment is prolonged antibiotic therapy. However, antibiotic-resistant strains of Brucella can be generated easily, and if such strains are used in bio-warfare, use of antibiotics to control brucellosis may not be effective. The Centers for Disease Control and Prevention has listed Brucella as a category-B biothreat-agent. Currently, there is no licensed vaccine against brucellosis in humans. Due to its highly infectious nature and the increased likelihood of illegitimate use, it would be beneficial to have available a vaccine that protects humans against this pathogen.
Several animal vaccines against different strains of Brucella currently exist.
Cattle vaccine strain RB51: This is an attenuated (less capable of surviving in animals, and less capable of causing disease in animals), rough (incapable of producing the cell-surface antigen called O-side-chain) strain of B. abortus. Strain RB51 induces strong cell-mediated immune (CMI) responses and provides protection against brucellosis in bovine and several other animal species. It is the official vaccine approved by USDA to protect cattle against infection with B. abortus. 
Although very effective in immunizing cattle against B. abortus, it is less effective against B. melitensis and B. suis infections, suggesting that strain RB51 would not be a suitable vaccine for humans, where B. melitensis and B. suis cause the most severe symptoms. The induction of O-side chain antibodies, in addition to strong CMI, appears to be important for protection against brucellosis in humans. Strain RB51 is rough, and therefore expresses only minimal amounts of O-side chain antigen and does not induce O-side chain antibodies.
In addition, strain RB51 was developed through natural selection procedures, and therefore, its genetic make up is not filly known. Further, strain RB51 is resistant to rifampicin, one of the very few antibiotics available for treatment of humans against brucellosis. Thus, if a human vaccinated with RB51 did become infected, (e.g. an immuno-compromised individual), it would not be possible to treat the infection with standard antibiotic therapy with rifampicin. For these reasons, strain RB51 is not considered a suitable candidate for use as a brucellosis vaccine for humans.
Cattle vaccine strain 19: This strain is able to induce protective immunity in cattle. However, although strain 19 (also known as S19) is of low virulence for cattle, vaccination of pregnant cows can still result in abortions. A less frequent adverse consequence of strain 19 vaccination is the development of an arthropathy associated with Brucella antigen-containing immune complexes (but not live organisms) in the affected joints.
Strain 19 is known to be pathogenic for human beings. In addition, this strain was isolated through laboratory selection procedures, and its genetic make up is not understood. Therefore, strain 19 is also not considered a suitable candidate for use as a brucellosis vaccine for humans.
Sheep/goat vaccine strain Rev1: Rev1 vaccine is a live, attenuated B. melitensis strain that stimulates protection against infection with B. melitensis in sheep and goats and also protects rams against infection with B. ovis. Depending on the dose administered during pregnancy, abortions will occur with variable frequency. In cattle, Rev1 gives better protection than strain 19. However, strain Rev1 is also not considered safe as a human vaccine because its genetic makeup is not known, it is pathogenic for human beings, and it is resistant to the antibiotic streptomycin.